Pat the ahi tuna dry with a paper towel and dice into 1/2-inch cubes.
In a small glass bowl, whisk together the tamari, toasted sesame oil, grated fresh ginger, lime juice, and rice vinegar to create the marinade.
Add the diced tuna to the marinade, tossing gently to coat, and let it marinate in the refrigerator for 10 minutes.
Prepare your base by placing the cooked brown rice in the center of a shallow bowl.
Thinly slice the cucumber and radishes, and dice the avocado.
Arrange the marinated tuna, edamame, cucumber, radishes, and avocado in sections over the brown rice.
Drizzle any remaining marinade over the bowl and garnish with sliced green onions and black sesame seeds.